I've a 2011 MBA, 2008 MBP and a Dell U2410 24" 1920x1200-native setup with zero problems, until I replace the aging MBP with rMBP. Now I get the same issue as the OP.
I've searched extensively, but "blurry images, washed-out colors, smudgy text" isn't very scientific/concrete so it's not easy to have a clear-cut case. On this same issue (I believe), I've found the following threads:
I'm trying to document the issue as much as possible, with photos taken off the display. (I've found screen capture doesn't work). They're posted at http://goo.gl/XLdMf
Further background
The same mini-DP to DP cable is used to connect MBA, rMBP (and my previous 2008 MBP) and have been in service since I've the Dell. No issues at all, except the rMBP.
I've tried HDMI-HDMI from rMBP to Dell. Same lousy display quality.
The Dell U2410 accepts DP, HDMI, DVI and VGA inputs.
On the monitor itself, I can press a few buttons to ask for display info. In both cases, the Dell reports it is displaying in 1920x1200 @ 60Hz. (So this rules out the problem of trying to display 1920x1080 on a 1920x1200 native display).
I've done a Factory Reset on the Dell, to no avail.
When the MacBookAir4,2 (2011 MBA) is connected, a range of resolutions is available:
- 1080p, 1080i, 720p, 480p, 1920x1200x 1600x1200, 1600x1000, 1344x1008, 1344x840, 1280x1024, 1280x960, 1280x800, 1024x768, 1024x640, 960x600, 800x600, 800x500, 720x576, 640x480
When MacBookPro10,1 (rMBP) is connected, only 3 choices present itself:
- 1920x1200, 1600x1000, 1280x800
Comparing the 2 Colors tab photos, notice how, when the rMBP is connected, the fonts have a white shadow all around the black fonts.
Comparing the 2 Display tab photos, look at the "1920" in the selected "1920x1200" option. Notice how, when the rMBP is connected, the bottom of the "1", "9", "2", "0" are smudged together